Development and differentiation in higher organisms are in part the result of cells interacting with and interpreting signals from the environment. These signals can be cellular and extracellular. The identity of these signals and their mechanism of action is still unknown and remains a fundamental question in developmental biology. The primary goal of this proposal is to identify any cellular interactions, molecules or factors in the extracellular environment of the primary mesenchyme cells of sea urchin embryos that are important for differentiation of the embryonic skeleton and analyze how these environmental factors regulate the program of differentiation at the molecular level. SPECIFIC AIMS 1. Examine the role of cell-cell interactions on the differentiation of micromeres nd primarymesenchyme cells of the sea urchin embryo. 2. Examine the effects of modification sof the extracellular matrix (ECM) environment in the embryo on differentiation of the primry mesenchyme. The will involve the use of inhibitors, drugs and analogs known to interfere with the synthesis, secretion and processing of various extracellular matrix macromolecules. Depending on the particular experimental design for Aims 1 and 2, differentation will be monitored by one or more of the following parameters. a. morphological appearance of spicules. b. transcription of a specific spicule matrix gene c. accumulation of a specific spicule matrix mRNA. d. synthesis of spicule matrix proteins. 3. Characterize the ECM components synthesized by micromeres and primary mesenchyme.